Ž .Wear 231 1999 265–271

Wear in cast titanium carbide reinforced ferrous composites under drysliding

V.K. Rai, R. Srivastava, S.K. Nath ), S. RayDepartment of Metallurgical Engineering, UniÕersity of Roorkee, Roorkee, UP 247 667, India

Received 19 November 1997; received in revised form 7 April 1999

Abstract

Ž .Wear in composites containing 8, 15 and 25 vol.% of titanium carbide TiC in a pearlitic matrix synthesized by solidificationprocessing, has been tested under dry sliding in a block on ring Timken wear testing machine under loads of 131.8, 187.5 and 254.5 Nand sliding velocities of 25.67=10y2, 41.07=10y2 and 56.47=10y2 mrs. The volume of wear varies linearly with the slidingdistance under various test conditions as predicted by Archard’s equation. The volume wear rate increases linearly with increase in load aspredicted by Archard’s equation, more rapidly in the composites containing lower volume fraction of carbide. With increase in TiCcontent, the volume wear rate reduces presumably due to higher wear resistance of TiC but the rate appears to reach a steady value athigher TiC content. The wear coefficient of the composite reduces with increase in TiC content in the composite. However, the extent ofreduction is more in case of composites with lower TiC content. When the individual contribution to the wear coefficient by theconstituents have been evaluated by a rule of mixture it appears that TiC has a negative wear coefficient which could be an artifactdeveloping due to application of a simple rule of mixture to a situation of complex interaction as it exists in composite. However, wearcoefficient of the matrix pearlite varied from 1.77 to 1.88=10y4 which is close to that observed by other workers q 1999 ElsevierScience S.A. All rights reserved.

Keywords: Ferro–TiC; Dry sliding wear; Archard’s wear coefficient; Composites; Wear rate; Sliding distance

1. Introduction

Ž .Iron-based composites containing titanium carbide TiChave emerged as a new class of engineering materials withtoughness and machinability of tool steels, alloy steels andsuperalloys combined with the hardness and wear resis-

w xtance of cemented tungsten carbide 1,2 . These compos-Ž .ites synthesized by powder metallurgy PrM are available

commercially and have been employed in a number ofapplications owing to their excellent abrasion wear resis-tance and abilities to overcome galling, withstand abrasiveimpact and retain properties at elevated temperatures. Inaddition, the composite has superior corrosion resistanceand an ability to keep a keen cutting edge. Feed screws,underwater pelletizer knives, draw punches, draw rings,pill dies, PrM compaction dies, mill guide rolls, work

) Ž .Corresponding author. Tel.: q91-0132-65722 0 ; Fax: q91-1332-73560; E-mail: vcrmt@rurkiu.ernet.in

rolls, seals, rotor fuel pumps and a number of otherw xproducts have been made out of these composites 2 .

Plasma-deposited TiC reinforced composites have a slidingwear resistance about 20% higher than WCr12wroCo andabout 100% higher than Stellite 6e coatings depending on

w xthe choice of matrix alloy 3 . A number of ferrous matrixalloys like tool steels, stainless steels and maraging steels

w xhave been used 2 . It is claimed that the round carbidegrains protect the matrix while the matrix securely grips

w xthe self-lubricating TiC mini spheres 3 .Synthesis of composites by PrM is relatively more

expensive and imposes size limitations on components.Therefore, efforts have been directed to make these com-posites by solidification processing which offers an oppor-tunity to generate the TiC particles in situ in the molten

w xalloy 4,5 . There is very little possibility of surface con-tamination of these carbide particles which may result inloss of strength due to weak interface coherency. TiC

Ž .particles crystallize in the cubic NaCl B1 structure, hasdensity of 4.90–4.93 grcm3 and hardness of 3200 kgrmm2

w x6 .

0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 99 00127-1

( )V.K. Rai et al.rWear 231 1999 265–271266

2. Experimental procedure

Cast iron weighing 6–7 kg and containing 3.82 wt.%carbon have been melted in a high frequency inductionfurnace at 16008C. Calculated amounts of titanium andgraphite are added to obtain the desired amount of TiC byreaction of already present carbon and added graphite withtitanium in the molten alloy. The melt containing carbideparticles are properly mixed and cast into green sandmould of size 15=15=150 mm3.

Metallographic specimens of size 15=15=20 mm3

are sectioned from cast ingots and the surface is polishedwith emery papers up to 4r0 grade and with polishingcloth containing alumina dispersion. Microstructure is ex-amined under an optical microscope and determination ofvolume fraction and particle size of TiC have been carriedout. Microhardness of carbides is measured by a Leitzmicrohardness tester under a load of 50 g. Vickers hard-ness of composites is determined under a load of 30 kg.X-ray diffraction analysis have been carried out to identifythe phase constituents in the composite.

Tribological properties of the composites have beeninvestigated using a Timken wear testing machine whichemployed a steel ring rotating against the test block heldby a load applied by a lever mechanism. The loss ofweight of the test block is determined at definite timeintervals. The steel ring is made of carburized steel hard-ened to HRC 60–62 and has a circumference of 154 mm.The unevenness of the ring is about "10 mm. The ringhas been cleaned before each test. The test blocks of size12=12=20 mm3 were polished up to 4r0 grade ofemery paper, cleaned and weighed with an accuracy of10y5 g. The surface is examined by profilometry. The ringand the test block are mounted on the testing machine andthe desired load is applied before the ring shaft is made tomove at a given rpm. The tests have been conducted tofind the variation of weight loss with time for a given loadand sliding velocity. After conducting the wear tests, thewear scars of the samples are examined by scanningelectron microscopy and profilometry.

3. Results and discussion

As it has been already demonstrated by several investi-w xgators 1,4,5 , the present study also confirms that it is

Fig. 1. Typical microstructure of Fe–TiC composite as observed underŽ . Ž .optical microscope at magnification a 125= and b 650=, enlarged

further 2.5 times.

possible to synthesize ferrous alloy base composites con-taining TiC generated in situ, by solidification processing.The physical characteristics of the Fe–TiC composites,synthesized in this study, are given in Table 1. The densityof the cast composites varies from 7.0 to 7.22=103

kgrm3 and a higher amount of carbide results in lowerrange of densities. TiC has a density of 4.94=103 kgrm3

and the matrix pearlitic steel has density of 7.84=103

kgrm3. The carbide particle sizes vary between 2 and 5mm. Fig. 1a and b show a typical microstructure of thecomposite observed under an optical microscope at amagnification 125= and 650= , respectively. The struc-ture reveals dark areas of pearlite in the matrix, resolved insome places at higher magnification and a number of smallwhite particles of TiC embedded in it. In addition, thereare relatively larger white areas of Fe Ti which also forms2

during processing. The average microhardness of the ma-

Table 1Physical characteristics of composites

3 3Ž . Ž .Composite number Volume percent of TiC particles vol.% Macrohardness VHN Density, 10 kgrm

Theoretical Experimental

I 8 500 7.61 7.22II 15 550 7.4 7.0III 25 550 7.11 7.18

( )V.K. Rai et al.rWear 231 1999 265–271 267

trix, Fe Ti and the TiC particles are observed as 500, 1520,2

and 2300 VHN, respectively. X-ray diffraction of thecomposite confirms presence of Fe Ti, TiC, Fe C and bcc2 3

solid solution of Ti in Fe.The composites containing 8, 15 and 25 vol.% of TiC

have been tested for dry sliding against a counterface ofcarburized steel of hardness HRC 60–62. The variation ofwear volume with sliding distance is shown in Fig. 2a, band c, for tests, carried out under loads of 131.8, 187.5 and254.5 N, respectively, and a fixed sliding velocity of25.67=10y2 mrs. It is observed that the variation inwear volume is almost linear, inspite of inhomogenities

expected in the cast samples. For a test load of 187.5 N,when the sliding velocity is increased to 41.07=10y2 or56.47=10y2 mrs, the variation of wear volume withtime still remains linear, as shown in Fig. 3a and b. Theslopes of linear variation have been determined from thevolume loss data by least square fit and the coefficients ofcorrelation in all the cases are found to be better than0.994. From the slope, the wear rate has been calculated asthe volume loss per unit sliding distance under differenttest conditions.

Fig. 4 shows the variation of wear rate with TiC contentin the three composites under investigation. It has been

Fig. 2. Variation of wear volume with sliding distance for Fe–TiC composites containing 8, 15 and 25 vol.% TiC, tested at a constant sliding velocity ofy2 Ž . Ž . Ž .25.67=10 mrs and at loads of a 131.8, b 187.5 and c 254.5 N.

( )V.K. Rai et al.rWear 231 1999 265–271268

Fig. 3. Variation of wear volume with sliding distance for Fe–TiC composites containing 8, 15 and 25 vol.% TiC, tested at a constant load of 187.5 N andŽ . y2 Ž . y2sliding velocities of a 41.07=10 and b 56.47=10 mrs.

observed that the wear rate reduces with TiC content up to25 vol.% under different loads used in the current investi-gation. But the wear rate appears to reach a steady valuewith higher TiC content. Fig. 5 shows that the variation ofwear rate with load is almost linear in the range of loadunder investigation, for all the three composites. For a

given load the extent of variation of wear rate with slidingvelocity is relatively small for all the composites as shownin Fig. 6 and the wear rate reduces with sliding velocity forcomposites containing 15 and 25 vol.% of TiC. But for thecomposite containing 8 vol.% TiC, there appears to be aminimum in the variation of wear rate with sliding veloc-

( )V.K. Rai et al.rWear 231 1999 265–271 269

Fig. 4. Variation of wear rate with composition for Fe–TiC composites tested at a constant sliding velocity of 25.67=10y2 mrs and loads of 131.8, 187.5and 254.5 N.

ity. However, this may not be taken as an established trendin view of the small variation of wear rate with slidingvelocity.

w xHolm 7 deduced the volume of wear, W, after slidingover a distance of d under load L as,

KLdWs 1Ž .

H

where H is the penetration hardness of the surface whichis wearing away and K is a nondimensional constant,

w xcalled wear coefficient. Archard 8 analyzed the processof sliding when junctions exist between sliding surfacesand their size is similar to those of the wear particlesgenerated. The volume of wear is the same as that given

Ž .by Eq. 1 except for the value of Archard wear coeffi-XŽ .cient, K s3K which is different in the Archard equa-

tion due to a geometrical factor of three in the denomina-tor. The linear variation of wear volume with time as

Fig. 5. Variation of wear rate with load for Fe–TiC composites contain-ing 8, 15 and 25 vol.% of TiC, tested at a constant sliding velocity of25.67=10y2 mrs.

shown in Figs. 2 and 3 implies also a linear variation ofŽ .wear volume with sliding distance as given by Eq. 1 .

Therefore, the composites under investigation obey Ar-chard’s equation for the range of load and sliding velocityused. Fig. 7 shows the variation of wear coefficient, K ,with composition. When the volume of carbide has in-creased from 8 to 25 vol.% the overall wear coefficient ofthe composite has reduced from 1.46=10y4 to 0.9=

y4 w x10 . The 1946 wear coefficient table of Holm 7 showthat for steel on steel the wear coefficient is 126=10y4

w xand the 1957 wear coefficient table of Hirst 9 shows thatfor tool steel on tool steel the wear coefficient is 1.3=

10y4. The cast Ferro–TiC composites with a higher TiCcontent appear to have wear coefficients superior to toolsteel on tool steel.

During dry sliding, the hard TiC particles or Fe Ti do2

not easily come out in the debris because of their reason-ably good bonding with the matrix. However, debonding

Fig. 6. Variation of wear rate with sliding velocity for Fe–TiC compos-ites containing 8, 15 and 25 vol.% of TiC tested at a constant load of187.5 N.

( )V.K. Rai et al.rWear 231 1999 265–271270

of some of these particles particularly at higher loads hasbeen observed. But the extent of abrasive three-body wearis considered relatively insignificant and the wear processis primarily adhesive.

The wear of the composite has been analyzed using asimple rule of mixture approach which presumes that theconstituents are responding independently to the wear pro-cess. This effort is directed to determine wear resistance ofeach constituent in the overall wear resistance in thecomposite. If one ignores the intermetallic phase of Fe Ti,2

the composites under investigation contain two con-stituents, i.e., TiC and the matrix pearlite with distinctlydifferent mechanical characteristics. Archard’s equationhas been derived for a single phase material originallyalthough it has been applied subsequently for multi-phasematerials. One may modify the Archard’s equation formaterials with two constituents using a rule of mixturewhich presumes that the load in both the constituents inthis case, TiC and pearlite, are proportional to their areafraction.

W K A L K A LTiC TiC P Ps q 2Ž .

d H HTiC P

where K and K are the wear coefficients of TiC andTiC P

pearlite, respectively, and A and A are their relativeTiC P

area fractions. H is the hardness of TiC measured asTiC

2300 kgrmm2 and H is the hardness of pearlite mea-P

sured as 500 kgrmm2. K and K have been calculatedTiC P

using the experimental results for the composites contain-ing 8 and 15 vol.% TiC and these values are shown inTable 2.

The wear coefficient of TiC appears to be negative,which is possibly an artifact developing due to applicationof a simple rule of mixture to a very complicated wearinteraction between TiC and pearlite. The scatter in thewear data may also be responsible for a misleading nega-tive wear coefficient of TiC. However, transfer of material

Fig. 7. Variation of Archard’s wear coefficient with TiC content inFe–TiC composites tested at a sliding velocity of 25.67=10y2 mrs.

Table 2Wear coefficients of TiC and pearlite

Ž .Load kg Wear coefficients

K KP TiC

y4 y313.44 1.77=10 y1.26=10y4 y319.11 1.88=10 y1.54=10

from the steel counterface to hard TiC areas may alsocontribute to a negative K . The wear coefficient of theTiC

pearlite matrix shows a very small variation with load, butthe value is of a magnitude, similar to ks1.48=10y4

w xobserved by Wang et al. 10 in pearlitic eutectoid steel.Leaving aside the uncertainty introduced by the formationof Fe Ti, it appears that the rule of mixture may be able to2

predict the wear coefficient of a constituent in a compositeat least under certain circumstances. The interaction of thematrix pearlite with an increased amount of TiC particlesshould have resulted in a reduced wear of the matrix butthis fact may have got suppressed by negative wear coeffi-cient of TiC. In view of the scatter observed commonly inthe results of wear it is difficult to assess the limitation ofapplicability of the rule of mixture.

4. Conclusions

The current study on wear of Ferro–TiC compositeleads to the following conclusions.

Ž .1 Following Archard’s equation, the volume of wearis linear with sliding time and sliding distance at appliednormal loads of 131.8, 187.5 and 254.5 N and slidingvelocities of 25.67=10y2 , 41.07=10y2 , and 56.47=

10y2 mrs with coefficient of correlation better than 0.994.Ž .2 The volume wear rate reduces with increase in TiC

content, presumably due to higher wear resistance of thiscarbide, but the rate appears to reach a steady value athigher TiC content.

Ž .3 The volume wear rate increases linearly with in-crease in load, more rapidly in the composite containing alower volume fraction of carbide. It is again conformingwith the prediction of Archard’s equation.

Ž .4 The wear coefficient of the composite reduces withincrease in TiC content in the composite. However, theextent of reduction is more in case of composites withlower TiC content.

Ž .5 When the individual contribution to the wear coeffi-cient by the constituents is evaluated by a rule of mixture,it appears that TiC has a negative wear coefficient, whichcan be an artifact of application of rule of mixture to thecomplex state of interaction as it exists in the composite.However, the wear coefficient of the matrix pearlite variedbetween 1.77 and 1.88=10y4 , which is close to thatobserved by others for pearlitic eutectoid steel.

( )V.K. Rai et al.rWear 231 1999 265–271 271

References

w x1 V.K. Rai, S.K. Nath, S. Ray, Wear behaviour of cast Fe–TiCcomposites, Proc. IXth ISME Conference on Mech. Eng., AjayPrinters and Publishers, Roorkee, India, 1994, pp. 407–412.

w x2 Unique solutions for tough wear problem, Brochure, Alloy Technol-ogy International, New York, 1994.

w x3 R.W. Smith, D. Gentener, E. Harzenski, T. Robisch, The structureand properties of plasma-sprayed TiC dispersion hardened coatings,Thermal Spray Technology—New Ideas and Processes, Proc. Na-tional Thermal Spray Conference, ASM International, OH, USA,1988, pp. 300–306.

w x4 T.Z. Kattamis, T. Suganuma, Solidification processing and tribologi-cal behaviour of particulate TiC–ferrous matrix composites, Mater.

Ž .Sci. Eng., A 128 1990 241–252.w x5 B.S. Terry, O.S. Chinyamakobvu, In situ production of Fe–TiC

composites by reactions in liquid iron alloys, J. Mater. Sci. Lett. 10Ž .1991 628–629.

w x6 P. Schwarzkopf, R. Kieffer, Refractory Hard Metals, Macmillan,New York, 1953, pp. 85–88.

w x7 R. Holm, Electric Contacts, Section 40, Almquist and Wiksells,Stockholm, 1946.

w x8 J.F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys. 24Ž .1953 981–988.

w x9 W. Hirst, Wear of unlubricated metals, Proc. Conf. Lubr. Wear,Ž .IME, London 1957 674–681.

w x10 Y. Wang, L. Pan, T.C. Lei, Sliding wear behaviour of pearliticŽ .structures in eutectoid steel, Wear 143 1991 57–69.

V.K. Rai received his Masters degree in Metallurgical Engineering withspecialization in Industrial Metallurgy in 1994 from the University ofRoorkee, India.R. Srivastava received his Masters degree in MetallurgicalEngineering with specialization in Industrial Metallurgy in 1995 from theUniversity of Roorkee, India.

Ž .S.K. Nath received the BE Metallurgy degree from University ofŽ .Roorkee in 1977, M. Tech Metallurgy from IIT Kanpur in 1979 and

PhD degree from University of Roorkee in 1990. Areas of interest includeferrous alloy design and development, structure–property correlation andtribological behaviour of ferrous alloys with the view to develop newalloys. Nath has been teaching in University of Roorkee since 1981.

Ž .S. Ray received the PhD degree in Metallurgical Engineering 1977 fromIIT Kanpur, worked in National Aeronautical Laboratory, Bangalore andNational Physical Laboratory, Delhi before joining University of Roorkeewhere he is serving as Professor since 1991. Ray worked as a visitingAssociate Professor at University of Wisconsin-Milwaukee, USA andlater as visiting Professor at Institute National Polytechnique de Grenoble,France. Ray worked on cast metal matrix composites for 3 decades andhas many pioneering contributions and patents. Current interest is incomposite processing and the materials aspect of tribology.